As one of the conventional techniques for backing up a copy source volume of a storage product or a computer, an OPC (One Point Copy) technique is known. When backing up data, the OPC technique makes a snapshot of the data existing at a predetermined time without the subsequent updates, etc. Specifically, the OPC technique is a technique to back up all the data of the copy source volume involved at the time of generating a backup.
The backup apparatus for executing the OPC technique, for example, upon receiving an OPC command from the user to produce an OPC, copies all the data of the copy source volume at the point in time when the OPC command is received. The backup apparatus stores the copied data (backup data) as a snapshot at the time point of receiving the OPC command. Specifically, the backup apparatus stores the backup data as a copy of the whole copy source volume in correspondence with the time point of receiving the OPC command and a generation (session information).
A technique called “SnapOPC” is known as an advanced function of the OPC technique described above. The SnapOPC technique is a technique for backing up only the data that is generated before the update in the part of the data of the copy source volume which will be updated.
According to the SnapOPC technique, a physical volume smaller in capacity than the copy source volume is acquired as a copy destination volume to which the data is actually copied. As a result, the capacity required for the copy destination can be reduced. In order to make it possible for a host to access a copy destination volume in a similar way to a copy source volume on the other hand, the host recognizes a logical volume having the same capacity as the copy source volume. The LBA of the physical volume is referred to as a physical LBA, and the LBA of the logical volume is referred to as a logical LBA.
According to the SnapOPC technique, as depicted in FIG. 9, the physical LBA and the logical LBA are defined for the copy destination volume. The copy destination volume for SnapOPC is referred to as the “Snap Data Volume” (SDV). Also, in the copy destination volume, the LBA visible to the host is referred to as a logical LBA, and the LBA physically constructed is referred to as a physical LBA. The logical capacity of the copy destination is equal to that of the copy source.
In a backup apparatus carrying out the SnapOPC technique, a copy session is set and host I/O processing is executed with the logical LBA, and data is recorded as a physical LBA in the SDV on a first-come-first-served basis. At this time, an area for the LBA conversion table to convert the logical LBA to the physical LBA is secured in the SDV (see FIG. 10). The LBA conversion table holds the entries, which are in data units generated during the copy operation, in an amount equal to the logical capacity. Assume, for example, that 1 LBA unit (data block) is 512 bytes and 16 LBAs are managed as one entry. Then, the LBA conversion table holds the number of entries equal to the total number of logical LBAs divided by 16. The backup apparatus carrying out the SnapOPC technique is adapted to acquire the start address of the physical LBA having the length of 16 LBAs from the corresponding logical LBA.
The backup apparatus, upon receiving an I/O request from the host, reads the LBA conversion table for all the I/Os in one read operation. In the case where the physical LBAs are sequential, the read/write operation is performed collectively. In this case, during the read operation of the LBA conversion table, other data accesses to the particular LBA range are excluded. In the case where an I/O accessing the same area occurs, therefore, waiting for the exclusion occurs and exclusion waiting time is generated (see FIG. 11).
In the conventional technique described above, a plurality of entries are read from the LBA conversion table for one I/O operation, and therefore, there is a problem in that waiting for the exclusion of other data accesses to the LBA conversion table frequently occurs.